1. Field of the Invention
The present invention generally relates to a wheel bearing unit, and more particularly to a long-Life wheel bearing unit for rotatably supporting a car wheel on the body of a motor vehicle.
2. Description of the Prior Art
FIG. 1 is a diagram of a wheel bearing unit for use with a motor vehicle, illustrating an example of a construction designed for use in a driving wheel. This wheel bearing unit is composed of a hub wheel 1, an outer joint member 3 of a constant velocity universal joint 2, and an axle bearing 4 that are assembled into a unit. Note that the outer joint member 3 of the constant velocity universal joint 2 is, with an axially extending stem portion 5 inserted through a through hole 6 of the hub wheel 1 and serrations 7 and 8 formed in the outer circumference of the stem portion 5 and the inner circumference of the through hole 6, respectively, coupled to the hub wheel 1 and secured with a nut 9 so as to achieve torque transmission therebetween.
The constant velocity universal joint 2 is composed of, in addition to the outer joint member 3, an inner joint member 11 attached to the edge of a shaft 10, a plurality of torque transmission balls 12 incorporated into the track grooves of the inner and outer joint members 11 and 3, and a retainer 13 interposed between the outer spherical surface of the inner joint member 11 and the inner spherical surface of the outer joint member 3 for retaining the torque transmission balls 12.
The wheel bearing unit is so constructed that the hub wheel 1 is rotatably supported on the axle bearing 4. A vehicle wheel (not shown) is fixedly attached to the hub wheel 1, and the axle bearing 4 is supported through a knuckle 14 on a suspension system (not shown) of the car body.
The axle bearing 4 employs a double row angular ball bearing structure. In this construction, an outer ring 15 has double rows of raceway surfaces 16 and 17 formed in its inner diameter surface. One raceway surfaces 18 is formed in the outer circumferential surface of the hub wheel 1, and the other 19 is formed in the outer circumferential surface of an inner ring 20 that is press-fitted to the outer circumference of the edge of the hub wheel 1. These raceway surfaces 18 and 19 are opposite to the raceway surfaces 16 and 17 of the outer ring 15, respectively. Between the raceway surfaces of the outer ring 15, and the hub wheel 1, the inner ring 20 are interposed double rows of rolling elements 21 and 22. The rolling elements 21 and 22 are held at circumferentially equally-spaced positions in the retainers 24 and 23, respectively.
The outer ring 15 has a body mounting flange 25 protrudingly formed in its outer circumference. The flange 25 has, in several positions around its surface, female threads 26. By screwing a bolt 27 on the female threads 26, the outer ring 15 is fixedly attached to the knuckle 14. Note that the axle bearing 4 is provided with a seal 28, for preventing accidental intrusion of foreign material from outside or leak of grease contained inside.
The hub wheel 1 is provided with a wheel mounting flange 29. The flange 29 has a hub bolt 30 attached at circumferentially equi-spaced positions thereof for fixing a vehicle wheel. Moreover, a brake rotor 31 is fixedly attached to the flange 29 with the hub bolt 30.
Incidentally, in conventional wheel bearing units, the hub wheel 1 and the outer ring 15 are commonly made of a medium carbon steel which raceways are induction hardened. Specifically, steel materials ranging from S40C steel to S58C steel (as defined in the Japanese Industrial Standard (JIS) G 4051), in particular, S53C steel (a medium carbon steel) is employed.
The reason why the S53C steel (a medium carbon steel) is employed as an exemplary steel material for the hub wheel 1 and the outer ring 15 is as follows. If the hub wheel 1 and the outer ring 15 are sized and shaped based on a plastic working technique, in the case where, if a high carbon steel is used, for example, SUJ2 steel that is defined as a high carbon chromium steel for bearing structural use in JIS G 4805, it is inevitable that the forgeability is markedly deteriorated.
However, the use of S53C steel cannot be said to be highly advantageous in terms of a service life level, as compared with the rotating fatigue life of SUJ2 steel. In particular, the inner-board and outer-board raceway surfaces formed in the outer diameter surfaces of the hub wheel 1 and the inner ring 20, and the inner diameter surface of the outer ring 15, respectively, tend to suffer from a comparatively short service life. To prevent the service life from being shortened, the bearing is designed to have a larger rated load capacity so as to maintain the service life at adequate levels. This, however, cannot be achieved without making the size and weight of the bearing unduly large.
An object of the present invention is to provide a wheel bearing unit in which the service life level is successfully improved without making the size and weight of the bearing unduly large. To achieve the above object, according to one aspect of the present invention, a wheel bearing unit for rotatably supporting a wheel on the body of a motor vehicle is provided with a rotary member, a fixed member, and double rows of rolling elements. The rotary member to which a wheel is attached has double rows of raceway surfaces formed therein. The fixed member is fixedly coupled to a mounting member located on the car-body side of the device, and has double rows of raceway surfaces formed therein so as to be opposite to the raceway surfaces of the rotary member. The double rows of rolling elements are interposed between the raceway surfaces of the rotary and fixed members, respectively. In this construction, at least the rotary member is made of a carbon steel containing 0.60 to 0.80 wt % of carbon, and, in a predetermined area thereof, a surface hardened layer is formed by a high-frequency quenching technique.
In the wheel bearing unit embodying the present invention, by forming at least the rotary member from a carbon steel containing 0.60 to 0.80 wt % of carbon, it is possible to obtain satisfactory processability (forgeability). This is because the amount of carbon contained in this carbon steel is smaller than that contained in SUJ2 steel, which is defined as a high carbon chromium bearing steel in JIS G 4805 (C: 0.95 to 1.10 wt %). Moreover, by forming a surface hardened layer in a predetermined area thereof by a high-frequency quenching technique, it is possible to suppress degradation of the hardness of the steel and thus secure sufficiently long rolling fatigue life. This makes it possible to improve the service life level without making the size and weight of the bearing unduly large.
The rotary member is made of steel containing: C: 0.70 to 0.80 wt % (exclusive of 0.80), Si: 0.50 to 1.0 wt %, Mn: 0.10 to 2.0 wt %, Cr: 0.40 to 0.95 wt %, Al: 0.050 wt % or less, 0: 0.0030 wt % or less, and the remainder being Fe and inevitable impurities. The use of such a steel material helps achieve further improvement of the processability and the rolling fatigue life. Moreover, of the double rows of raceway surfaces of the rotary member, the inboard raceway surface is formed in an inner ring that is separately provided. The inner ring is made of an alloyed steel containing: C: 0.8 to 1.2 wt %, Si: 0.4 to 1.0 wt %, Cr: 0.2 to 1.2 wt %, and Mn: 0.8 to 1.5 wt %. The inner ring is, after undergoing carbonitriding treatment, quenched at temperatures in the range of 830 to 870xc2x0 C., and subsequently tempered at temperatures in the range of 160 to 190xc2x0 C., with the residual austenite content in the surface layer part being 25 to 50 vol.%.
Furthermore, the inboard raceway surface is formed in a separately provided inner ring that is made of a carburized steel containing 0.15 to 0.40 wt % of carbon. The raceway surface is composed of a surface hardened layer that contains carbon in the amount of 0.8 wt % or more and has Rockwell hardness of HRC 58 or more, and a core part that has Rockwell hardness of HRC 48 or more and below HRC 58. In the surface hardened layer, the residual austenite content is kept in the range of 25 to 35 vol. %, the grain size of the residual austenite is kept at 5 xcexcm or less, and residual carbide content is kept at 10% by area or less.
Of the double rows of raceway surfaces of the rotary member, the inboard raceway surface tends to suffer from a comparatively short service life. In view of this fact, the separately provided inner ring in which the inboard raceway surface is formed is made of a steel material having the above-described composition, and is subjected to the above-described heating treatment. By doing so, the rolling fatigue life can be further improved.
The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.